Serene Innovations Doorbell Alert CA-DXBrand NewIncludes One Year WarrantyThe Serene Innovations CADX CentralAlert&trade Door Chime / Intercom and Door Knocking Sensor detects vibration when someone rings your door bell. It sends a wireless signal to trigger the CA360 to alert you. You can directly attach the knocking sensor to the inside surface of your door to detect the knocking on your door. It comes with a 2.5mm jack so that you can directly connect to your existing doorbell/door chime.